1. Field of The Invention
This invention relates to a method of producing high specific surface area silica (SiO.sub.2) of uniform porosity suitable for liquid chromatography uses by hydrolysis of silicon tetrafluoride (SiF.sub.4) or silicon tetrachloride (SiCl.sub.4).
2. Description of Related Art
Silica gel is widely used in chromatographic separations. Silica gel having high specific surface area and uniform porosity is especially useful in chromatographic techniques. Because impurities interfere with chromatographic separations, suitable silica gel desirably is essentially pure. Suitable silica must also have high strength so that it is not crushed under high pressures utilized for high performance liquid chromatographic separations.
Silica gel used in chromatographic separations must have uniform properties, not only within one chromatography column, but also from column to column, to ensure reproducibility of results. Thus, it is not sufficient to have uniform properties merely within a single batch of silica gel. Properties of silica gel must be uniform and predictable from batch to batch. The cost must also be reasonable.
High specific surface area silica gel produced using known techniques is unsatisfactory in one or more of these aspects. For example, silica gel produced by hydrolysis of tetraethoxysilane (TEOS) is expensive because TEOS is a relatively expensive reactant. Acidulation of sodium silicate does not involve expensive starting materials, but produces a silica gel product which contains large quantities of contaminants such as unreacted silicates. The level of impurities is sufficient to make the silica gel unsuitable for use in chromatographic separations.
U.S. Pat. No. 3,236,594 discloses a process for making finely divided silica from silicon tetrafluoride. The process is operated at ordinary temperature and fluorine is recovered as an inorganic fluoride. Silicon tetrafluoride is absorbed into a lower aliphatic alcohol to form a solution of a stable complex. This solution is reacted with inorganic base chosen from the group consisting of ammonium hydroxide and the oxides and hydroxides of alkali metals and alkaline earth metals and at least a stoichiometric amount of water to hydrolyze the silicon tetrafluoride. Then, silica is separated from the mixture containing metal fluoride or ammonium fluoride. However, silica gel produced by this method is unsatisfactorily powdery and is not suitable for use in high performance liquid chromatography columns.
Liquid chromatography columns using silica gel typically are either normal phase or reversed phase. Normal phase liquid chromatographic separations utilize interactions between polar silanol moieties on the surface of the column packing and the composition being separated. In reversed phase liquid chromatography, a non-polar stationary phase is bonded to the silanol group on the surface of the silica gel. This stationary phase, in combination with terminal groups ("endcaps") reacted onto the stationary phase, interact with the composition being separated.
The properties of silica gel suitable for normal phase use, described above, are similarly desirable for silica gel used as substrate for reverse phase liquid chromatography columns. However, additional characteristics are necessary for reverse phase liquid chromatography column packing. To ensure reproducibility of result, the stationary phase should be normally distributed, i.e., distributed in a monolayer, over the surface of the silica gel.
Various stationary phases have been bonded on the surface of silica gel particles. Octadecylchlorosilanes are often used as the stationary phase. Organochlorosilanes of other carbon numbers (e.g., C.sub.2, C.sub.4, and C.sub.8) also are used for surface bonding. Use of monochlorosilane compounds typically ensures monolayer distribution of the stationary phase over the silica gel substrate.
The consistency of quality required of silica gel used in chromatographic columns requires control over product properties. It is difficult to control product characteristics, however, in known processes requiring gelation of silicon tetrachloride in which a large stoichiometric excess of water is used. Further, these processes require long gelling periods, and the specific surface area of silica thus produced is too high for chromatographic uses.
It is an object of this invention to provide a method for producing high specific surface area silica gel by hydrolysis of silicon tetrahalide, i.e., silicon tetrafluoride or silicon tetrachloride.
It is another object of the invention to provide a method for obtaining silica gel having a selected specific surface area and narrow pore diameter range.
It is a further object of this invention to provide a high specific surface area silica gel suitable for use in high performance liquid chromatography columns.
It is yet another object of this invention to provide a method of preparing material which can be bonded to prepare packing material for reversed phase high performance liquid chromatography columns.